The present invention is directed to a system for transmitting data between a plurality of data processing units and more particularly to a communication controller which includes a circuit for generating signals acknowledging the receipt of a data message from a sending data processing unit.
In data processing units in which a central processing unit or multiple processing units are connected over a common communication channel to a plurality of peripheral devices, data may be transmitted over the communication channel between one of the central processor units and one of the peripheral devices, or between a sending and receiving peripheral device, or between processing units. As part of the communication protocol, the receiving device is required to transmit data to the sending device acknowledging receipt of the data. The simplest acknowledged message comprises a sequence of binary 0's or binary 1's. The transmitting or sending device then detects the presence of the transitions on the communications channel and assumes that the message has been received. However, it has been found that noise present on the communication channel could look like an acknowledgment to the sending device even though the message might be in error. Another type of acknowledgment message that is commonly used comprises a standard data message which normally includes opening and closing flags, an acknowledgment status character and a check character to ensure that the status character is properly received. It is obvious that the use of this type of message construction requires an increase in the length of the response time of the data transmissions between the sending and receiving devices. It is therefore a principal object of this invention to provide a data transmission system in which the acknowledgment message of a receiving device is in a form which provides a maximum amount of information with a minimal amount of transmitted data bits, and provides a high degree of detection by the sending data processing device.